Aqueous compositions containing dispersed active components are employed in a large number of industrial processes for the surface treatment of components and semi-finished products, wherein the components and semi-finished products are usually contacted with the aqueous dispersion by spraying or dipping. The aqueous dispersions that are contacted with the components and semi-finished products for surface-finishing purposes can for the most part be applied multiple times before the dispersion becomes so depleted in active components that additional active components have to be added for the desired success of the treatment. The aqueous dispersions that are employed for surface treatment are therefore collected after contacting with the component or stored in dipping baths and optionally worked up and regenerated so that they can be applied again for surface treatment. This type of repeated or continuous application leads to contaminants that adhere to the components being taken up by the aqueous dispersions and concentrated over time. Particularly critical is the salination of the aqueous dispersions, i.e. the entrainment of salts, since this causes a significant reduction in the stability of the dispersed active components. With increasing ionic strength, dispersions tend to coagulate since the electrostatic repelling forces between similarly charged electrolytic double layers of the dispersed active components are significantly weakened, so that the thermodynamically favored coagulation is inhibited less strongly. The progressive coagulation of the dispersed active components creates technical problems, on the one hand because sedimented active components are no longer available for surface finishing and on the other hand because the result of the surface finishing often depends partly on the size distribution of the dispersed active components.
For instance, the activation of vehicle bodies prior to anti-corrosive phosphating can be achieved with aqueous colloidal compositions of sparingly soluble phosphates or titanium salts, which form crystallization nuclei on the metal surfaces for the subsequent phosphating. Activation baths of this type are described in EP 0977908 or in WO 94/029495. The ability to form crystallization nuclei is possessed by the active components only if they do not exceed a particular particle size. In automotive manufacturing, vehicle bodies are contacted with the activating solution in the phosphating line, usually by spraying or dipping, during which contaminants, in particular salts, are introduced into the activating solution by the vehicle body. The entrainment of certain unwanted polyvalent ions, e.g. from the metal body to be treated, into these activation baths for phosphating can lead to more rapid consumption of the activating solution as a result of the associated acceleration of coagulation, and so this solution has to be either disposed of prematurely or regenerated by adding concentrates. This type of specific salination or entrainment of unwanted ions can therefore only be tolerated within a specific concentration range.
To a particular degree, the stability of organic binder dispersions is also influenced by the introduction of polyvalent ions. For example, aluminum salts are often employed as auxiliary substances for separating off coating components in the wet scrubbing of paint spray booths in the automotive industry. In many applications, however, the emphasis is on maintaining the stability of aqueous binder dispersions. For instance, after the anti-corrosive phosphating, vehicle bodies are generally provided with a dip coating. The dip coating is a water-based dispersion of one or more organic binders, which are deposited on the body upon immersion in the dip coating either electrolessly or with the application of an external voltage. The deposition mechanism of the binder in the dip coating is based on the fact that the surface charge of the dispersed binders in the immediate vicinity of the surface of the vehicle body is removed, so that coagulation and deposition of binder particles take place. Dip coatings therefore tend to coagulate as a result of their application if the concentration of a particular ionic species is exceeded. Conversely, this means that the entrainment or concentrating of certain ions can markedly reduce the stability of the dip coating. Typically, anodic or autophoretic dip coatings substantially consist of anionically stabilized binder components which can coagulate rapidly in the presence of polyvalent cations.
A need therefore exists to regulate the ionic strength or concentration of certain polyvalent ions in aqueous dispersions of inorganic solids and/or organic binders in order to maintain the stability of the dispersions over a prolonged period. To this end, it is necessary to establish a method that brings about a selective depletion of polyvalent cations in aqueous dispersions, but without significantly reducing the proportion of dispersed active components.
Various established separating methods exist for this purpose in the prior art, in which a selective mass transfer takes place through membranes. It is certainly generally known to the person skilled in the art that he can utilize filtration with exclusion limits lying just below the average particle size of the dispersed active components in order to separate the active components from the aqueous phase. However, so-called “cake-forming” filtration methods are not suitable for obtaining a filtrate containing the excess or unwanted polyvalent ions, since the filter cake consisting of the retained active components can only be redispersed with great effort. Membrane filtration methods that are operated as cross-flow filtration are suitable in principle for obtaining a filtrate, wherein unwanted ions can also be selectively removed from the retentate using ion-exchanging membranes. However, the aqueous dispersion is subjected to high shear stresses in so-called cross-flow filtrations, leading to coagulation of the dispersed constituents in the filter unit and in the pumps. This phenomenon often occurs in dispersions of organic binders, so the cross-flow filtration of e.g. dip coatings cannot be used for reducing ionic strength. Common to the filtration methods is the fact that the excess or unwanted polyvalent ions must first be removed from the filtrate, e.g. by precipitation, before the filtrate can be fed back to the dispersion, resulting in an overall reduction of the ionic strength or of the concentration of certain polyvalent ions in the dispersion. The use of filtration methods for reducing ionic strength is therefore extremely complex from a process engineering point of view, even in cases in which sufficiently high stability of the dispersion towards shear stress exists.
A method of reducing ionic strength in resin dispersions, which is suitable for the autodeposition of organic coatings on metallic surfaces, preferably iron surfaces, without external current can be taken from DE 3431276. The reducing of ionic strength, in particular of the concentration of iron ions, is carried out here too in order to stabilize the binder system in an autophoretic bath against coagulation. DE 3431276 gets around the problem of high shear stresses leading to coagulation of the paint constituents by recirculating the filtrate, i.e. the aqueous phase that is to be enriched with the unwanted ions, while the dispersion itself does not flow over ion-exchanging membranes in the filter module and is only in contact with these. This method makes possible a controlled reduction of the ion load in the aqueous dispersion of the binder, but the filter module must be regularly backwashed to remove binder particles from the membrane surfaces in order to guarantee consistent separating performance.
WO 02/18029 discloses a method for the selective separation of polyvalent cations from an autophoretic bath containing an organic binder dispersed in water, which tends to coagulate rapidly in the presence of polyvalent cations of the elements iron and zinc. According to this method, an acidic selective cation exchanger is supplied with the bath that is enriched with polyvalent cations and thus a reduction of the ions is achieved. The cation exchanger has to be regenerated at regular intervals.